1. Field of the Invention
The present invention relates to semiconductor integrated circuit (IC) manufacturing, and more specifically, to a set of two phase-shifting masks with sub-wavelength diffractive optical elements and a method of designing such a set of two phase-shifting masks with sub-wavelength diffractive optical elements.
2. Discussion of Related Art
Improvements in photolithography have allowed higher density and faster speed to be attained in Integrated Circuits (ICs) by continually shrinking the devices in a chip. According to the Rayleigh criterion, the minimum Critical Dimension (CD) which can be resolved by an imaging tool may be directly proportional to the wavelength of the light source and inversely proportional to the Numerical Aperture (NA) of the projection optics. However, diffraction may degrade the aerial image when the CD becomes smaller than the wavelength of the light used to expose a photoresist film on a wafer. The exposure light may include deep ultraviolet light with a wavelength of 248 nanometers (nm) or 193 nm.
Photolithography in the sub-wavelength regime will benefit from wavefront engineering using a resolution enhancement technique (RET), such as a phase-shifting mask (PSM), to achieve a sufficiently wide process latitude. Unlike a conventional binary mask, such as chrome-on-glass (COG), that only modulates amplitude of light, a PSM also modulates phase of the light to beneficially use destructive interference to mitigate the detrimental effects of diffraction.
An alternating phase-shifting mask (AltPSM) may help to pattern a feature with a small linewidth or critical dimension (CD), such as a gate length of a transistor for a device in the chip. AltPSM improves contrast between exposed and unexposed regions of the photoresist film by introducing a phase difference of 180 degrees between the light transmitted through adjacent clear apertures of the mask to force amplitude of light between the corresponding two images to zero.
However, even an AltPSM may be unable to provide sufficient pattern fidelity as the CD is scaled down. Consequently, a variety of techniques may be needed to enhance fidelity of the pattern printed on the wafer.
In particular, a technique known as optical proximity correction (OPC) may be used to modify the features in the patterns on the mask to compensate for variations and non-uniformities in the fabrication process for the layer of the chip.
When traditional OPC is applied to the design of the mask, certain sub-resolution assist features, such as serifs and anti-serifs, may be used to modify the edges of the product features. Other sub-resolution assist features, such as scattering bars, may also be added nearby to the product features.
Empirically-derived rules may be formulated for OPC to help define product features that otherwise cannot be reliably reproduced during manufacturing. However, as the features shrink in size, many conflicts may arise in applying the rules. Thus, a mask, such as a PSM with OPC, may become very complex to design.
Thus, what is needed is a set of two phase-shifting masks with sub-wavelength diffractive optical elements and a method of designing such a set of two phase-shifting masks with sub-wavelength diffractive optical elements.